The applicant is a nephrologist with an interest in transplant immunology, who is eligible for and fulfills all the necessary requirements to be considered for a Mentored Clinical Scientist Award. As part of his nephrology fellowship at the Brigham and Women's Hospital and The Children's Hospital in Boston, the applicant completed 3 years of laboratory-based research during which he gained practical experience in techniques of cellular and molecular immunology. The main thrust of his research has been the study of the role of P-glycoprotein, the multidrug resistance-associated ABC transporter, in alloimmunity, and the principle findings are now being published in the Journal of Immunology. The applicant is now completing his nephrology fellowship and will be appointed as a staff member at the Brigham and Women's Hospital and as an Instructor of Medicine at Harvard Medical School effective July 1, 2001. In this proposal, the applicant wishes to build on his findings that P-glycoprotein serves critical roles in vitro in T cell activation and APC function. He has found that specific P- glycoprotein blockade inhibits alloantigen- but not mitogen-dependent T cell activation by inhibiting IFN-gamma and IL-12 through a positive feedback loop, thus blocking a critical pathway in the initiation of primary alloimmune responses. These mechanisms may also be operative in vivo and P-glycoprotein may therefore play an important role in clinical allograft rejection. However, the in vivo role of P-glycoprotein in T cell activation and allorecognition has not been explored. In addition, the molecular signals and mechanisms that regulate P-glycoprotein expression by T lymphocytes in the course of alloimmune interactions are currently unknown. The goals of this proposal are to: 1) define the role of P-glycoprotein in the direct and indirect pathways of allorecognition and in allograft rejection in vivo in murine models of allotransplantation; 2) investigate the in vivo function of P-glycoprotein in antigen-specific T cell activation using a murine adoptive transfer system where a small population of peptide-specific CD4+ TCR transgenic T cells can be physically tracked and phenotypically examined; and 3) examine in vitro the role of P-glycoprotein in antigen presenting cell function and in the bi- directional activation of T cells and allogeneic endothelium which is critical for allograft rejection, based on the preliminary identification of a unique capacity of allogeneic endothelium to trigger T cellular P-glycoprotein expression. The applicant's training in molecular biology and cellular immunology has allowed him to discover a novel role of P-glycoprotein in alloimmunity. The applicant realizes that in order to further explore his recent findings he needs additional mentoring. The Mentored Clinical Scientist Development Award will provide the critical opportunity for the applicant to further expand his knowledge in immunology necessary for achieving independence. The Mentor's and the Co-Mentor's laboratories provide an ideal environment to accomplish the aims of this project because of their strong background in transplantation immunology, their collaborative momentum and their exemplary didactic and ethical curricula. After completing this project the applicant will be familiar with the critical in vitro and in vivo techniques of molecular and cellular immunology in order to examine independently and competitively the role of related or novel molecular targets in transplantation and allograft rejection.